Examples of reflectance sensing include bar code sensing, edge detection, and reflective position encoding. Bar code technology is well established and provides an accurate, easy and inexpensive method of data storage and data entry for computerized information management systems. Various optical scanning systems have been developed for reading bar codes appearing on a label or on the surface of an article. Generally a bar code is a series of lines (bars) and spaces having various widths. The light reflecting characteristic of a line is different to that of a space. The information is encoded as width variations. The most common type of data stored in a bar code is item identification information, used for inventory control, process tracking, distribution tracking and other material management functions. In these applications, the bar code may represent a product number, serial number or encoded description of the item. Bar codes are also used where information about an item or a transaction must be accurately entered into a computer.
Optical scanners are used to detect the bar code and transform the bar code information into electrical signals, which are then decoded into alphanumeric characters.
A number of different scanning systems have been developed for bar code reading, these include laser scanners, charge coupled device (CCD) scanners and scanners using light emitting diodes (LED) and photodiodes.
An LED bar code scanner typically consists of an LED and a photodiode packaged with suitable electronics and focusing optics. The optical elements focus the LED light as a spot that has a width less than or equal to the smallest line width in a product barcode. The reflected light is received by the photodiode and the presence or absence of light at the photodiode is detected.
LED scanners may be compact and lightweight and may be used in a variety of applications including bar code scanning, edge sensing and positioning and optical tachometers.
Since an LED emits radiation over an extremely large aperture angle, optical lenses are used when it is desirable to focus the radiation on small area. Optical lenses have been used with LED's in optical sensors and for coupling light to optical fibers. These lenses are used with the LED positioned on the optical axis of the lens.
Lenses are also used to focus light from an illuminated object onto a photodiode. Again, these lenses are used with the photodiode positioned on the optical axis of the lens.